Movable micromachined silicon structures frequently are used as sensors and actuators, and for signal processing. For example, the fingers of a movable mass may be positioned between stationary plates to form a differential capacitor. When a force is applied to the structure in a predetermined direction, the movable mass moves relative to the plates, changing the capacitances and inducing an amplitude modulated signal onto the mass. The induced signal, which is proportional to the magnitude of the acceleration, may be output for further processing. An example of such a device is described in U.S. Pat. No. 5,345,824, entitled "Monolithic Accelerometer", which is incorporated herein by reference.
In designing a micromachined electro-mechanical structure, it is important to minimize or stabilize parasitic electrostatic forces on the structure. For example, placing a conductive plane under the structure that is kept at nearly the same electrical potential as the structure minimizes the electrostatic attractive force that would tend to pull the structure down to the substrate. However, in the conventional fabrication process for this sensor, in order to conduct further processing, the conductive plane is coated with a thin, insulating dielectric layer. The presence of this thin dielectric layer results in an electrically isolated surface underneath the micromachined structure. Electric changes can accumulate on the surface of the dielectric layer and cause electrostatic forces, indistinguishable from an inertial force such as one produced by acceleration, on the structure. This results in a shift in the sensor output. Also, the potential of this isolated surface changes due to charge accumulations, which causes a spurious output signal.
The present invention overcomes these problems as will be shown in the remainder of the specification referring to the attached drawings.